This invention relates generally to well logging apparatus and, more specifically, to methods and apparatus for releasing arm members of a well logging instrument in response to temperature.
It is common in the well logging industry to employ instruments having one or more arm members which can be urged against the borehole walls. Such instruments include, for example, centralizers, having a plurality of arm members which when urged against the borehole walls cause the main body member to be centered in the borehole; calipers, where the expansion of the arm members is translated into a signal indicative of the diameter of the borehole; and various logging instruments, which require one or more measuring elements to be placed in contact with the borehole wall.
Difficulty may occur when lowering such instruments into a borehole with the arm members in an expanded position. Such an operation can result in breakage or excessive wear to the arms. Additionally, when attempting to lower such an instrument with expanded arm members into a deviated borehole the friction of the armborehole wall contact can prevent the instrument from traversing downward into the borehole.
In an effort to overcome these difficulties, prior art in the well logging field has relied on various devices such as motorized actuators, explosive devices and hydraulic solenoid devices. While such devices have met with varying degrees of success they suffer from practical limitations. Motorized actuators add size, weight, cost and complexity to the instrument. Explosive ignitors, such as one used to separate a tensile bar for arm release, are complex, unreliable and unsafe.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a simple, economical, reliable means for retaining mechanical arm members in a closed position while a well logging instrument is lowered into a borehole and releasing the arm members at an approximate predetermined location within the borehole.